Threshold is the point that there is lactate acumulation but still in control, if you go higher it increases at a much rapid pace, and it differs from people, middl edistance runners have higher lactate threshold than distance runners in mmol.
there are many ways to see where it is aproximatly. hr%, lactate test using fix value, oxigen consumption, estimations based on performace and so on.
The way it is tested at the higher level is in 2 parts first a incremental test increasing speed each mile or so, while measuring lactate each rep, hr and oxigen, then plot the graph and clearly there is a point where lactate spikes you get that as reference, then in a future sesion you do a 20min continuos efort at that pace and lactate should be stedy if not its lower, if oits stedy you can retest again another session higher speed and repeat, depend on the level of acuracy you need, 99% of runners with invremental test is more than enough and there you can also see hr at given intensity.
Thing to have in consideracion is that hr can have a lot of variation in tearms of heat (rn is summer), hidration, fatigue, and so one so pair it with feeling is good
Also if you look at diferents groups how they train at threshold some tend to push it more to the limit and some go more conservative. So in practice go at a pace where the breathing is control, the intesnity 6-7/10 and that you are not forcing the body.